The process of firing transformation of ceramic billet is controlled by chemical reaction. All ceramics are beyond the control of the whole process. The basic principle is that when the temperature reaches 573 ℃, the lattice structure outside the ceramic crystal is destroyed, and the water molecules of the structure water are separated from the minerals, and a series of changes have taken place in the volume of the silica in the ceramic billet. The temperature gradually increased, and the ceramic billet was finally burnt and mature.
Due to the differences in material selection and process of different ceramic types, the firing of each kind of ceramic has its own unique features based on this basic firing principle, and the firing of Jianshui purple pottery has its own characteristics.
The final product of Jianshui purple pottery mainly depends on the firing atmosphere. When there is a big fire in the kiln, a completely different environment will be formed in the kiln.
The content of iron in Jianshui purple pottery mud is high, and the iron oxide is rich in the ceramic body; the kiln is in combustion state, and carbon reacts with oxygen to form carbon oxide. These two substances are basically fixed, so one of the main variables affecting the firing effect is the control of oxygen content in the firing process.
During the process of firing, the airlock is open and oxygen is sufficient. The iron elements in the ceramic billet are fully combined with oxygen, and finally all iron oxide is produced. The color is red, that is, the finished color of purple pottery polished after oxidation and burning.
When the temperature in the kiln increases, the air lock will be closed gradually, so as to control the oxygen supply in the kiln and the reduction atmosphere will be formed gradually. In the strong reducing atmosphere, it can be observed that the flame licks the kiln mouth, which is the flame grabbing oxygen for combustion.
At this time, the whole furnace is in anoxic state, and the carbon element in carbon oxide will grab the oxygen element in iron oxide to help maintain combustion, thus stripping iron oxide into elemental iron. After the whole reduction reaction is completed, a layer of simple iron is attached to the surface of the finished purple pottery, and the color is black gray or black purple, which is the black purple purple pottery works we see after grinding.
Finally, I would like to say that the shrinkage rate of Jianshui purple pottery is high, which is caused by two stages in the firing process
The first is that when the temperature rises to about 573 ℃, the water in the final crystal structure of the ceramic body is discharged, and the ceramic body becomes a dry body without any water. The loss of water makes Jianshui purple pottery shrink for the first time.
Second, Jianshui purple pottery clay has a variety of components, their melting points are different; Jianshui purple pottery sintering temperature is more than 1000 ℃, under such a high temperature, the low melting point material melts into a liquid state and fills between the high melting point particles, the ceramic body shrinks, the density of ceramic body increases, and becomes compact and solid.
Therefore, we say that Jianshui purple pottery is “as hard as iron”. In fact, when the optimum density is reached, the ceramic body will be compact and firm; if not, it will be loose in texture, dull in color, even cracked and deformed, and become waste. Therefore, it is called the art of taking treasure from fire.
In addition, the “sound like Qing” of Jianshui purple pottery is mainly for the works of reducing burning. The reduction firing makes the purple pottery produce simple iron, so it is knocked like a chime, and the work of oxidation burning has no effect. You can choose a pot in red and black, open the lid, hit the pot with your fingers and hear the difference.